The present invention relates in general to a tape drive calibration meter and pertains, more particularly, to a digital display calibration meter for calibrating magnetic tape recording equipment to digitally display such parameters as azimuth head alignment (skew) and tracking.
Digital magnetic tape recording and readout equipment is extensively used as a computer peripheral for information interchange with the computer. In order to render the recording medium compatible with different tape drives, the tape drive, such as one handling the standard half inch magnetic tape, must have the capability of recording data at the prevailing data packing density and track spacing with sufficient accuracy. In particular, the recorded data has to be capable of being reliably recorded when the recording medium is read out on equipment other than that on which it was recorded. As packing densities increase, greater performance demands are placed on the equipment used for data recording as well as data readout. Present day commercially available equipment is supposed to be capable of producing recording medium that is compatible throughout the industry. However, the degree of compatibility depends largely on the calibration procedures, the equipment that is used, such as calibration tapes, and the accuracy of measurements of the parameters that are involved.
The degree with which compatability can be maintained between magnetic tape recording equipment depends upon the following parameters:
1. Azimuth alignment and gap scatter of magnetic recording-reproducing heads, usually measured by using a master head alignment tape or skew tape;
2. Accuracy of tracking or positioning of magnetic heads, usually measured by using a tracking or positioning tape;
3. Amplitude or gain adjustment of read amplifiers, usually measured by using a master output tape; and
4. Velocity and velocity variations of tape drives, usually measured by any one of several methods and a variety of different pieces of equipment.
Reference is now made to FIGS. 1-3 which illustrate further background with respect to present techniques used to measure the above parameters. These techniques generally require the use of calibration tapes and an oscilloscope to display the waveforms of the readout signals. FIG. 1 shows some of these signals while FIG. 2 illustrates the oscilloscope arrangement.
The azimuth alignment of a magnetic recording-reproducing head is a mechanical adjustment and is usually most readily achieved by the use of electronic measurements. FIG. 1A illustrates the mechanical skew that occurs between the magnetic head 10 and the tape 12 passing in the direction of the tape path illustrated in FIG. 1A. FIG. 1B illustrates a form of read/write skew in which the outside tracks are in proper orientation being perpendicular to the tape path but the individual tracks 11 are not in a straight line. FIG. 1C shows typical read bus signals with skew present. FIG. 1C illustrates a leading track in solid by curve 13 and a lagging track in dotted by curve 14.
FIG. 2 shows a prior art connection of read amplifiers along with an oscilloscope for displaying the waveforms of the outside tracks showing the skew or time difference between peaks which are being measured. FIG. 2B illustrates the proper position without skew in which all of the peaks align while FIG. 2C shows the signals from the outside tracks such as track 9 in comparison with the reference signal shown in solid as, for example, track 1. In the example that is given, the magnetic tape is considered as being a nine track tape for use with a nine track head. In FIG. 2C the track 9 is represented by dotted lines in different waveforms spread on both sides of the perpendicular line X.sub.0. FIG. 2C illustrates the dynamic skew or jitter of the outside track about the reference which is to be measured with accuracy in order to align the head accurately. Visually, the extreme position of the positive peaks of the waveform are located at (+x) and (-x). These values can be added algebraically to obtain the average position of the head or average skew. Presently, these measurements typically yield poor results, prolonging the time required to accomplish head alignment.
FIG. 3 illustrates the tape tracking calibration. As in the skew tests, the tracking or head positioning test presently involves the reading of signals of the center track, measuring the amplitude of the two peaks, and calculating the mistracking of the head. FIG. 3 shows the tape 12 illustrating from a schematic standpoint, the backplate 16 and the head 10 in different positions identified as positions 10A, 10B, and 10C. In position 10A the head is in a correct tracking position as noted by the waveforms. Positions 10B and 10C illustrate the head in incorrect tracking positions. In position 10B the head must be moved toward the backplate 16 to correct the misalignment while in the position 10C the head must be moved away from the backplate 16 to correct the misalignment. Although this method of verifying and measuring tracking or head positioning is adequate, improved means are described in accordance with the present invention as discussed in detail hereinafter.
With respect to amplitude tests as presently taken, amplitude measurements and gain adjustments involve measurements of readout signals derived from a master signal level tape and displayed on an oscilloscope. With the use of the oscilloscope as illustrated in FIG. 2A, the reading of signals is quite difficult. For example, to read a signal on the order of 10 volts is quite difficult on an oscilloscope and it is most difficult to obtain accuracies any better than + or -5%. This is about the maximum limit of tolerance when gains are adjusted.
Generally, one of the last tests that is performed in the calibration procedure is the velocity of tape motion test. The velocity of tape motion is effected by mechanical tolerances of the capstan and the driving circuits of the capstan motor. Techniques for velocity measurements are somewhat undefined. There are several methods by which these measurements may be made. However, each method requires different types of equipment which are not of standard type and which are not readily carried by field service people in the field. As a result, velocity tests are rarely performed and when velocity problems develop in the field, the solution to these problems is generally very costly. Each method that is presently used for velocity measurement requires more than one piece of equipment and therefore the sources of error increase as a function of the multiplicity of pieces of equipment that are used.
For example, to measure the velocity of tape on a transport, it is necessary to first measure the diameter of the capstan by a mechanical device. The capstan is coated with some type of suitable rubber which is soft. Readings of the capstan periphery with a periphery gage can easily produce a 5% error which is twice the tolerance allowed for speed variation of the tape. Second, the revolutions per second of the capstan are measured in order to calculate the velocity of the tape. This measurement may be done optically with an optical tachometer or a mechanical tachometer which is required to be in contact with the shaft of the capstan motor. Either of these tachometer tools have inherent inaccuracies and when their error is multiplied by the error due to the diameter of the capstan, the net result may be intolerable. Therefore, these measurements have to be done several times to obtain an average reading. This makes the task time consuming in addition to being highly inaccurate.
Analysis of existing tape drive calibration techniques shows that the resolution of measurements taken on the oscilloscope varies depending upon the individual doing the test and the degree of his or her experience in the field. Also, the results of the calibration vary from one individual to another as well as the length of time it takes to complete the calibration.
Accordingly, it is an object of the present invention to provide a tape drive calibration meter which is adapted in essence to replace the presently existing oscilloscope reading technique and to instead display a number of different pertinent parameters on a readily readable digital display such as an LCD display.